Technical Field
Embodiments disclosed herein relate generally to the placement and spacing of rotatable cutting elements on a downhole cutting tool.
Background Art
Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and cutter pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation.
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.
PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
PDC cutters have been used in industrial applications including rock drilling and metal machining for many years. In PDC bits, PDC cutters are received within cutter pockets, which are formed within blades extending from a bit body, and are typically bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
In a typical application, a compact of polycrystalline diamond (PCD) (or other ultrahard material) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
A PDC cutter is conventionally formed by placing a sintered carbide substrate into the container of a press. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate and treated under high pressure, high temperature conditions. In doing so, metal binder (often cobalt) migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn integrally bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide-cobalt. The deposited diamond layer is often referred to as the “diamond table” or “abrasive layer.”
An example of a prior art PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIGS. 1A and 1B. The drill bit 200 includes a bit body 210 having a threaded upper pin end 211 and a cutting end 215. The cutting end 214 typically includes a plurality of ribs or blades 220 arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body 210. Cutting elements, or cutters, 250 are embedded in the blades 220 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle and side rake angle against a formation to be drilled.
A plurality of orifices 216 are positioned on the bit body 210 in the areas between the blades 220, which may be referred to as “gaps” or “fluid courses.” The orifices 216 are commonly adapted to accept nozzles. The orifices 216 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 220 for lubricating and cooling the drill bit 200, the blades 220 and the cutters 250. The drilling fluid also cleans and removes the cuttings as the drill bit 200 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 250 may result in cutter failure during drilling operations. The fluid courses are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 200 toward the surface of a wellbore (not shown).
Referring to FIG. 1B, a top view of a prior art PDC bit is shown. The cutting face 218 of the bit shown includes six blades 220. Each blade includes a plurality of cutting elements or cutters generally disposed radially from the center of cutting face 218 to generally form rows. Certain cutters, although at differing axial positions, may occupy radial positions that are in similar radial position to other cutters on other blades.
Cutters are conventionally attached to a drill bit or other downhole tool by a brazing process. In the brazing process, a braze material is positioned between the cutter and the cutter pocket. The material is melted and, upon subsequent solidification, bonds (attaches) the cutter in the cutter pocket. Selection of braze materials depends on their respective melting temperatures, to avoid excessive thermal exposure (and thermal damage) to the diamond layer prior to the bit (and cutter) even being used in a drilling operation. Specifically, alloys suitable for brazing cutting elements with diamond layers thereon have been limited to only a couple of alloys which offer low enough brazing temperatures to avoid damage to the diamond layer and high enough braze strength to retain cutting elements on drill bits.
A significant factor in determining the longevity of PDC cutters is the exposure of the cutter to heat. Conventional polycrystalline diamond is stable at temperatures of up to 700-750° C. in air, above which observed increases in temperature may result in permanent damage to and structural failure of polycrystalline diamond. This deterioration in polycrystalline diamond is due to the significant difference in the coefficient of thermal expansion of the binder material, cobalt, as compared to diamond. Upon heating of polycrystalline diamond, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the polycrystalline diamond. Damage may also be due to graphite formation at diamond-diamond necks leading to loss of microstructural integrity and strength loss, at extremely high temperatures.
Exposure to heat (through brazing or through frictional heat generated from the contact of the cutter with the formation) can cause thermal damage to the diamond table and eventually result in the formation of cracks (due to differences in thermal expansion coefficients) which can lead to spalling of the polycrystalline diamond layer, delamination between the polycrystalline diamond and substrate, and conversion of the diamond back into graphite causing rapid abrasive wear. As a cutting element contacts the formation, a wear flat develops and frictional heat is induced. As the cutting element is continued to be used, the wear flat will increase in size and further induce frictional heat. The heat may build-up that may cause failure of the cutting element due to thermal mis-match between diamond and catalyst discussed above. This is particularly true for cutters that are immovably attached to the drill bit, as conventional in the art.
Accordingly, there exists a continuing need to develop ways to extend the life of a cutting element.